They don't need to retool, they just need hardware that can run the games they make with their existing tooling.
Related, I invite you to review the comments and note how many are focused on the battery life of the Deck, and specifically on how short it is. Even if Nintendo could just upgrade to a faster CPU, that would bring increased power draw, which would lead to either shorter battery life or needing a bigger battery, at which point you are either at people complaining about your battery life or once again back to industrial design and retooling your assembly line.
Tldr: They can't just bump the price by $200, drop in a Threadripper, and call it a day.